And I'll bet that both of them were undereating, whether they realize it or not. It's very hard to get in enough calories on a low carb plan if you're muscular and active; I used to have to eat massive amounts of peanut butter to do it (nearly a jar a day, iirc, in addition to real food), and I was still at a caloric deficit. I'm on a fairly low carb plan now, averaging about 50 net g carbs/day, and I do between one and two hours of moderate exercise a day, and never get tired. I'm now adding in more strenuous cardio, and so far have had no problems. I've been doing this for a year, so maybe it takes a long time for your body to adapt. It is doable, though. BTW, many moons ago I used to train with a guy who got his IFB pro card, and he followed a low carb plan, eating almost nothing but steak, shrimp, vegetables, and protein shakes. But he ate a lot of them, in addition to the other, ummm, "aids" he was taking.